A Detailed Study of Cu(In,Ga)Se 2 Thin Films by Electron-Beam-Induced-Current and Cathodoluminescence
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A Detailed Study of Cu(In,Ga)Se2 Thin Films by Electron-Beam-Induced-Current and Cathodoluminescence M.J. Romero, F.S. Hasoon, M.M. Al-Jassim, R.Garcia1, and R. Noufi National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393 Phone: 303-384-6653, Fax: 303-384-6604, Email: [email protected] 1 Universidad de Cadiz, Cadiz, Spain ABSTRACT Cu(In,Ga)Se2 (CIGS) thin films were deposited using the three-stage process. At the third stage, an amount of Indium was added to the CIGS that is greater than the standard used in processing high-efficient CIGS solar cells. The effects of Indium excess and substrate temperature were then investigated by electron-beam-induced-current (EBIC) and cathodoluminescence (CL). The addition of more indium compared to the standard noticeably affects the ZnO/CdS/CIGS heterojunction. On the other hand, the substrate temperature primarily affects the luminescence behavior of these films. It is suggested than In enrichment and Na incorporation play a main role in the electronic properties of the film. From these results, the efficiencies obtained for this set of CIGS cells are finally understood. INTRODUCTION Cu(In,Ga)Se2 absorbers are the most promising candidates for high-efficiency terrestrial photovoltaics. Although the efficiency of CIGS solar cells has approached 19% [1], further improvements require a better understanding of the absorber properties and their impact on cell performance. Furthermore, the quaternary Cu-In-Ga-Se system, with the possibility of several phases and unequal incorporation of In and Ga in the absorber, complicates the understanding of these films. Most of the absorbers for high-efficiency cells have been fabricated with the threestage process [2]. The initial stage consists of the deposition of In-Ga-Se precursors. They are then exposed to Cu and Se fluxes in the second stage to form Cu-rich CIGS. In the final stage, slightly In-rich CIGS thin films are obtained by the addition of In, Ga, and Se. In this work, the In flux through the third stage has been maintained longer that the standard time used in processing high-efficiency CIGS solar cells. The effects of Indium excess and substrate temperature were then investigated by electron-beam-induced-current (EBIC) and cathodoluminescence (CL). The addition of indium affects substantially the ZnO/CdS/CIGS heterojunction and the carrier collection mechanism of the cell, whereas the temperature has a beneficial effect on the diffusion length. On the other hand, the substrate temperature mainly affects the luminescence behavior of these films, with enhanced non-radiative recombination at grain boundaries and improved spectral purity. EXPERIMENTAL DETAILS (In,Ga)2Se3 precursors were deposited on soda lime glass substrates at 400°C. Through both the second and third stages the temperature was set at either 570°C or 580°C. The equivalent In thickness deposited at the third stage, being 400 Å in the standard process, was increased to 1120 Å (an In excess of 720 Å) for a total set of five films. The substrat
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